U.S. patent application number 10/057310 was filed with the patent office on 2002-08-22 for semiconductor laser diode and optical communication system.
Invention is credited to Kinoshita, Junichi.
Application Number | 20020114369 10/057310 |
Document ID | / |
Family ID | 18884859 |
Filed Date | 2002-08-22 |
United States Patent
Application |
20020114369 |
Kind Code |
A1 |
Kinoshita, Junichi |
August 22, 2002 |
Semiconductor laser diode and optical communication system
Abstract
A surface emitting laser that provides an improved temperature
characteristic, an improved optical output, and easy forming
process. An optical communication system using this surface
emitting laser is also provided. A surface emitting laser
comprising: a substrate; a vertical cavity of layers formed on said
substrate for propagating and resonating light along an axis
vertical to a surface of said substrate, said light emitted from an
active layer by current injection; and a reflective film disposed
concentrically with the vertical axis around the outer periphery of
said vertical cavity for reflecting the light from said active
layer in a horizontal direction parallel to the surface of said
substrate, the light emitted from said active layer forming a laser
beam due to resonation, which laser beam is then emitted in a
vertical direction. An optical communication system using the
surface emitting laser.
Inventors: |
Kinoshita, Junichi; (Grove,
IL) |
Correspondence
Address: |
HOGAN & HARTSON L.L.P.
500 S. GRAND AVENUE
SUITE 1900
LOS ANGELES
CA
90071-2611
US
|
Family ID: |
18884859 |
Appl. No.: |
10/057310 |
Filed: |
January 25, 2002 |
Current U.S.
Class: |
372/46.01 ;
372/96 |
Current CPC
Class: |
H01S 5/1032 20130101;
H01S 5/2027 20130101; H01S 5/2063 20130101; H01S 5/1071 20130101;
H01S 5/18344 20130101; H01S 5/026 20130101; H01S 5/18313 20130101;
H01S 5/04254 20190801; H01S 5/18308 20130101; H01S 5/18333
20130101 |
Class at
Publication: |
372/46 ;
372/96 |
International
Class: |
H01S 005/00; H01S
003/08 |
Foreign Application Data
Date |
Code |
Application Number |
Jan 26, 2001 |
JP |
2001-018901 |
Claims
What is claimed is:
1. A surface emitting laser comprising: a substrate; a vertical
cavity of layers formed on said substrate for propagating and
resonating light along an axis vertical to a surface of said
substrate, said light emitted from an active layer by current
injection; and a reflective film disposed concentrically with the
vertical axis around the outer periphery of said vertical cavity
for reflecting the light from said active layer in a horizontal
direction parallel to the surface of said substrate, the light
emitted from said active layer forming a laser beam due to
resonation, which laser beam is then emitted in a vertical
direction.
2. The surface emitting laser according to claim 1, wherein the
light from said active layer are reflected by said reflective film
to thereby resonate also in the horizontal direction.
3. The surface emitting laser according to claim 1, wherein said
vertical cavity has a second-order or more optical periodicity
along the vertical axis, and said light take the form of a
radiation mode that propagates within said vertical cavity.
4. The surface emitting laser according to claim 3, wherein said
vertical cavity further has a discontinuity in a phase of said
periodicity.
5. The surface emitting laser according to claim 3, wherein said
periodicity is asymmetrical along the vertical axis.
6. The surface emitting laser according to claim 1 further
comprising: a phase modulator provided between said vertical cavity
and said reflective film for controlling the phase of the
light.
7. The surface emitting laser according to claim 1, wherein said
vertical cavity comprises said active layer.
8. The surface emitting laser according to claim 1, wherein said
active layer is provided between said vertical cavity and said
reflective film.
9. An optical communication system comprising: a surface emitting
laser comprising: a substrate; a vertical cavity of layers formed
on said substrate for propagating and resonating along an axis
vertical to a surface of said substrate, said light emitted from an
active layer by current injection; and a reflective film disposed
concentrically with the vertical axis around the outer periphery of
said vertical cavity for reflecting the light from said active
layer in a horizontal direction parallel to the surface of said
substrate, the light emitted from said active layer forming a laser
beam due to resonation and being emitted in a vertical direction;
an optical fiber for transmitting the laser beam from said surface
emitting laser therethrough; and a photodetector for receiving the
laser beam from said optical fiber and for converting the laser
beam to an electrical current.
10. A surface emitting laser comprising: a substrate; a first DBR
formed on said substrate and taking the form of a cylinder having a
central axis vertical to a surface of said substrate and exhibiting
a high reflectivity to light having a wavelength of .lambda.; a
first conductive type cladding layer formed on an overall surface
of said first DBR; an active layer formed on an overall surface of
said first conductive type cladding layer for emitting light having
a wavelength of .lambda. by current injection; a second conductive
type cladding layer formed on an overall surface of said active
layer; a second DBR taking the form of a cylinder formed on said
second conductive type cladding layer and smaller in radius than
said first DBR, said second DBR having the same central axis as
said second conductive type cladding layer, said second DBR
exhibiting a high reflectivity to the light having a wavelength of
.lambda.; said first DBR, said first conductive type cladding
layer, said active layer, said second conductive type cladding
layer, and said second DBR composing a vertical cavity; a burying
layer of a second conductive type formed on said second conductive
type cladding layer around the outer periphery of said second DBR,
said burying layer exhibiting a lower refractive index than said
second DBR; a reflective film covering the outer peripheries of
said first DBR, said first conductive type cladding layer, said
active layer, said second conductive type cladding layer, and said
burying layer, said reflective film exhibiting a high reflectivity
to the light having a wavelength of .lambda.; the light from said
active layer are resonated in a horizontal direction due to
reflection of the light by said reflective film and focused in the
vicinity of said central axis, the focused light are also resonated
in a vertical direction by said vertical cavity to thereby form a
laser beam, which is then extractable in the vertical direction
from said second DBR.
11. The surface emitting laser according to claim 10 further
comprising: a first electrode for injecting a current via said
first conductive type cladding layer into said active layer; and a
second ring-like electrode formed on said burying layer for
injecting a current into said active layer via said burying layer
and said second conductive type cladding layer.
12. The surface emitting laser according to claim 10, wherein said
substrate is made of GaAs, each of said first and second DBRs
comprising alternate layers of AlGaAs and AlAs, each of said first
and second cladding layers being made of AlGaAs, said active layer
being made of GaAs, said burying layer being made of GaAlAs.
13. The surface emitting laser according to claim 10, wherein said
reflective film is made of an insulator.
14. The surface emitting laser according to claim 11, wherein said
second electrode comprises a plurality of ring-like subelectrodes
disposed concentrically.
15. A surface emitting laser comprising: a substrate; a cylindrical
vertical cavity waveguide formed on said substrate and having a
central axis vertical to said substrate, said waveguide comprising
a lamination of alternate layers of different refractive indexes
for causing light having a wavelength of .lambda. to resonate in a
vertical direction; a cladding layer formed around the outer
periphery of said vertical cavity waveguide on said substrate, said
cladding layer exhibiting a smaller refractive index than an
average refractive index of said vertical cavity waveguide; a
conductive area formed around the outer periphery of said cladding
layer on said substrate, said conductive area comprising a first
conductive type cladding layer, an active layer formed on said
first conductive type cladding layer for emitting light having a
wavelength of .lambda. by current injection, and a second
conductive type cladding layer formed on said active layer; a
reflective film covering the outer periphery of said conductive
area and exhibiting a high reflectivity to the light having a
wavelength of .lambda.; the light from said active layer are
resonated in a horizontal direction and also in a vertical
direction by said vertical cavity to thereby form a laser beam,
which is then extractable in the vertical direction from said
vertical cavity waveguide.
16. The surface emitting laser according to claim 15 further
comprising: a first electrode for injecting a current via said
first conductive type cladding layer into said active layer; and a
second ring-like electrode formed on said conductive area for
injecting a current into said active layer via said second
conductive type cladding layer.
17. The surface emitting laser according to claim 15, wherein said
substrate is made of GaAs, said vertical cavity waveguide
comprising alternate layers of AlGaAs and AlAs, each of said first
and second cladding layers is made of AlGaAs, and said burying
layer is made of GaAlAs.
18. The surface emitting laser according to claim 15, wherein said
reflective film is made of an insulator.
19. The surface emitting laser according to claim 15, wherein said
vertical cavity waveguide comprises a phase shifter flush with said
active layer for shifting by .lambda./4 the phase of light having a
wavelength of .lambda. from said active layer.
20. The surface emitting laser according to claim 16, wherein said
second electrode comprises a plurality of ring-like subelectrodes
disposed concentrically.
Description
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] This application is based upon and claims the benefits of
priority from the prior Japanese Patent Application No.
2001-018901, filed on Jan. 26, 2001, the entire contents of which
are incorporated herein by reference.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The present invention relates generally to a semiconductor
laser diode and an optical communication system.
[0004] 2. Related Background Art
[0005] Various light elements such as light emitting elements,
photodetectors and light modulating elements are used in an
extensive field and placed as basic devices that support an
information technology society. Among those light elements, the
semiconductor lasers exhibit a narrow spectrum of light emissions,
are coherent and can focus light energy of an extremely high
density on a small area. Thus, the semiconductor lasers have found
applications in various fields such as optical communication,
medical care, display devices reading/writing from/to optical
discs, etc.
[0006] In the above semiconductor lasers, many edge emitting lasers
that emit laser beams parallel to substrates thereof are used.
Surface emitting lasers that emit laser beams vertical to the
substrates thereof are also used. Since the surface emitting lasers
involve surface emissions, they are suitable for being constituted
as two-dimensional arrays and also for coupling to optical
fibers.
[0007] Vertical cavity surface emitting lasers (VCSELs) that have
laser cavities extending vertical to the substrates thereof have a
merit that they operate at a low threshold current, and they have
been greatly studied recently. The VCSELs, however, have a drawback
that they exhibit no excellent high-temperature characteristic and
provide a low optical output although they operate at a low
threshold current.
SUMMERY OF THE INVENTION
[0008] According to embodiments of the present invention, there is
provided a surface emitting laser comprising:
[0009] a surface emitting laser comprising:
[0010] a substrate;
[0011] a vertical cavity of layers formed on said substrate for
propagating and resonating light along an axis vertical to a
surface of said substrate, said light emitted from an active layer
by current injection; and
[0012] a reflective film disposed concentrically with the vertical
axis around the outer periphery of said vertical cavity for
reflecting the light from said active layer in a horizontal
direction parallel to the surface of said substrate,
[0013] the light emitted from said active layer forming a laser
beam due to resonation, which laser beam is then emitted in a
vertical direction.
[0014] According to embodiments of the present invention, there is
provided an optical communication system comprising: An optical
communication system comprising:
[0015] a surface emitting laser comprising: a substrate; a vertical
cavity of layers formed on said substrate for propagating and
resonating light along an axis vertical to a surface of said
substrate, said light emitted from an active layer by current
injection; and a reflective film disposed concentrically with the
vertical axis around the outer periphery of said vertical cavity
for reflecting the light from said active layer in a horizontal
direction parallel to the surface of said substrate, the light
emitted from said active layer forming a laser beam due to
resonation and being emitted in a vertical direction;
[0016] an optical fiber for transmitting the laser beam from said
surface emitting laser therethrough; and
[0017] a photodetector for receiving the laser beam from said
optical fiber and for converting the laser beam to an electrical
current.
[0018] According to embodiments of the present invention, there is
further provided a surface emitting laser comprising:
[0019] a substrate;
[0020] a first DBR formed on said substrate and taking the form of
a cylinder having a central axis vertical to a surface of said
substrate and exhibiting a high reflectivity to light having a
wavelength of .lambda.;
[0021] a first conductive type cladding layer formed on an overall
surface of said first DBR;
[0022] an active layer formed on an overall surface of said first
conductive type cladding layer for emitting light having a
wavelength of .lambda. by current injection;
[0023] a second conductive type cladding layer formed on an overall
surface of said active layer;
[0024] a second DBR taking the form of a cylinder formed on said
second conductive type cladding layer and smaller in radius than
said first DBR, said second DBR having the same central axis as
said second conductive type cladding layer, said second DBR
exhibiting a high reflectivity to the light having a wavelength of
.lambda.;
[0025] said first DBR, said first conductive type cladding layer,
said active layer, said second conductive type cladding layer, and
said second DBR composing a vertical cavity;
[0026] a burying layer of a second conductive type formed on said
second conductive type cladding layer around the outer periphery of
said second DBR, said burying layer exhibiting a lower refractive
index than said second DBR;
[0027] a reflective film covering the outer peripheries of said
first DBR, said first conductive type cladding layer, said active
layer, said second conductive type cladding layer, and said burying
layer, said reflective film exhibiting a high reflectivity to the
light having a wavelength of .lambda.;
[0028] the light from said active layer are resonated in a
horizontal direction due to reflection of the light by said
reflective film and focused in the vicinity of said central axis,
the focused light are also resonated in a vertical direction by
said vertical cavity to thereby form a laser beam, which is then
extractable in the vertical direction from said second DBR.
[0029] According to embodiments of the present invention, there is
further provided a surface emitting laser comprising:
[0030] a substrate;
[0031] a cylindrical vertical cavity waveguide formed on said
substrate and having a central axis vertical to said substrate,
said waveguide comprising a lamination of alternate layers of
different refractive indexes for causing light having a wavelength
of .lambda. to resonate in a vertical direction;
[0032] a cladding layer formed around the outer periphery of said
vertical cavity waveguide on said substrate, said cladding layer
exhibiting a smaller refractive index than an average refractive
index of said vertical cavity waveguide;
[0033] a conductive area formed around the outer periphery of said
cladding layer on said substrate, said conductive area comprising a
first conductive type cladding layer, an active layer formed on
said first conductive type cladding layer for emitting light having
a wavelength of .lambda. by current injection, and a second
conductive type cladding layer formed on said active layer;
[0034] a reflective film covering the outer periphery of said
conductive area and exhibiting a high reflectivity to the light
having a wavelength of .lambda.;
[0035] the light from said active layer are resonated in a
horizontal direction and also in a vertical direction by said
vertical cavity to thereby form a laser beam, which is then
extractable in the vertical direction from said vertical cavity
waveguide.
BRIEF DESCRIPTION OF THE DRAWINGS
[0036] FIG. 1 is a schematic perspective view of a surface emitting
laser according to a first embodiment of the present invention.
[0037] FIG. 2 is a schematic perspective view of a surface emitting
laser according to a second embodiment of the present
invention.
[0038] FIG. 3 is a schematic perspective view of a surface emitting
laser as a modification of the second embodiment.
[0039] FIG. 4 is a graph illustrating an asymmetrically formed
distribution of refractive indexes in a periodic al structure
42.
[0040] FIG. 5 schematically illustrates an optical communication
system according to a third embodiment of the present
invention.
[0041] FIG. 6 is a schematic perspective view of a VCSEL structure
made experimentally by the inventor in a course to the present
invention.
DETAILED DESCRIPTION OF THE INVENTION
[0042] Surface emitting lasers according to the embodiments of the
present invention and their optical communication systems will be
described with reference to the accompanying drawings.
[0043] Referring to FIG. 6, a VCSEL made experimentally by the
inventor will be described as a premise of the present invention.
The VCSEL of FIG. 6 is made of GaAlAs/GaAs materials and oscillates
at a 860 nm wavelength. A structure of the VCSEL will be outlined
in its making process as follows.
[0044] First, a lower DBR (Distributed Bragg Reflector) 102
composed of alternate layers of AlGaAs and AlAs is grown on an
n-type GaAs substrate 101. Then, formed on the lower DBR 102 are an
n-type AlGaAs cladding layer 103, GaAs active layer 104, a p-type
AlGaAs cladding layer 105, and an upper DBR 106 comprising a
lamination of alternate layers of p-type AlGa and AlAs.
[0045] Then, this product is etched out downward to the lower DBR
102 to form a vertical cavity (a cylindrical mesa) 110. The
cylindrical mesa 110 is then subjected to a vapor oxidizing process
on its side to thereby form Al oxide layers (insulators) 115 in the
vicinity of an exposed mesa side of the AlAs layers of the upper
DBR. By limiting the formation of the oxide layers 115 due to the
Al oxidation so as to leave a more central part of the wall of the
cylindrical mesa 110, a structure having a narrow central current
path to the active layer 104 is provided.
[0046] A p-side ring-like electrode 120 is formed on top of the
mesa 110 so that light are extracted from the central opening in
the upper surface of the mesa. An n-side electrode 121 is formed on
the back of the substrate 101. Thus, the VCSEL of FIG. 6 is
completed.
[0047] In the VCSEL of FIG. 6, a threshold current is small.
However, in the VCSEL of FIG. 6, the current is focused on a very
small active layer volume, and the VCSEL exhibits a bad
high-temperature characteristic and cannot provide large optical
output. Since the active layer into which a current is to be
injected is placed below the light extracting surface, a very
narrow ring-like electrode and an internal narrow current limiting
structure formed, for example, by selective oxidation are needed.
The narrow ring-like electrode will increase contact resistance.
High contact resistance and high current resistance (narrow current
path) will lead to a further deterioration in the temperature
characteristic. With the FIG. 6 structure, the p-side electrode 120
should be formed with high positional precision on top of the thin
cylindrical mesa 110 so as not to interrupt the laser beam
concerned and care must be taken to form the thin films and at PEP
process.
[0048] Keeping in mind the FIG. 6 VICSEL, just described above, the
embodiments of the present invention will be described next in
which a first and a second embodiment each involves a surface
emitting laser and a third embodiment involves an optical
communication system using the surface emitting laser.
[0049] (First Embodiment)
[0050] As will be seen in FIG. 1, one of the features of the
surface emitting laser of the first embodiment in that light
emitted from the active layer 4 are cross resonated based on
vertical resonation by vertical cavities 2-6 and horizontal
resonation by a reflective film 100 so as to provide a strong laser
beam in a direction of a vertical axis L of the laser.
[0051] FIG. 1 is a schematic perspective view of a surface emitting
laser in the first embodiment. The FIG. 1 laser is made of
GaAlAs/GaAs materials and oscillates at a wavelength of .lambda.,
which is 860 nm. A lower or first DBR 2 comprising alternate layers
of AlGaAs and AlAs, an n-type AlGaAs cladding layer 3, a GaAs
active layer 4, a p-type AlGaAs cladding layer 5, and an upper or
second DBR 6 comprising alternate layers of p-type AlGaAs and AlAs,
formed in this order on an ntype GaAs substrate 1 with a common
central axis L vertical to a surface of the substrate. The lower
and upper DBRs 2 and 6 each exhibit a high reflectivity to light
having a wavelength of .lambda. emitted from the active layer 4.
Therefore, the lower DBR 2, n-type cladding layer 3, active layer
4, p-type cladding layer 5 and upper DBR 6 constitute a vertical
cavity that causes light emitted from the active layer 4 by current
injection to propagate and resonate along the axis L. A cylindrical
mesa 10 is formed so as to cover the upper DBR 6 and a part of the
thickness of the p-type AlGaAs cladding layer 5. The cylindrical
mesa 10 has a smaller radius than the lower DBR around the common
central axis L. The mesa 10 is buried in a grown GaAs burying layer
30. The composition of the GaAs burying layer 30 is adjusted so as
to be lower in refractive index than the upper DBR 6. The burying
layer 30 is etched out so as to provide a large cylindrical mesa
11. The cylindrical mesa 11, lower DBR 2, n-type cladding layer 3,
active layer 4, p-type cladding layer 5 and burying layer 30 are
covered on their outer peripheries with a high reflection insulator
film 100, which exhibits a high reflectivity to light having a
wavelength of .lambda. from the active layer 4. The p-side
ring-like electrode 20 is provided on top of the burying layer 30
that constitutes a part of the cylindrical mesa 11. An n-side
electrode 21 is provided on the back of the substrate 1.
[0052] When a forward voltage is applied across the electrodes 20
and 21 to the pn-junction of the surface emitting laser of FIG. 1,
a current is injected into the active layer 4 to cause the active
layer 4 to emit light having a wavelength of .lambda. from the
overall surface of the active layer 4. The light from the active
layer 4 are reflected by the high reflection film 100 so that
resonation occurs in a horizontal direction parallel to a surface
of the substrate 1, and focussed in the vicinity of the vertical
axis L. More specifically, when the high reflection film 100 around
the outer periphery of the mesa has a reflectivity high to some
extent, the Q of resonation in the radius, horizontal direction of
the cylinder increases to focus the light in the vicinity of the
central axis L. These focussed light are also resonated in a
vertical direction by the vertical cavities 2-6. That is, in the
surface emitting laser of FIG. 1, "cross resonation"occurs due to
resonations in the radius vector or horizontal direction and in the
vertical direction of the cylindrical mesa 11. Strong light
obtained by this resonation can be extracted in the vertical axis L
direction from an opening in the upper surface of the DBR 6.
[0053] Since the light element of the present embodiment operating
based on the above-mentioned mechanism has a larger volume of the
active layer 4 than the VICSEL of FIG. 6, it exhibits a good
temperature characteristic and provides high optical output.
[0054] The ring-like p-side electrode 20 has a larger size than
that of the FIG. 6 VCSEL to thereby greatly reduce the contact
resistance. The p-side electrode 20 can be provided with a less
strict positional accuracy on the top of the thicker cylindrical
mesa 11.
[0055] Although not shown, one of a pair of ring-like subelectrodes
may be disposed substantially concentrically in the mesa instead of
the single ring-like electrode 20 so that any one of the pair of
subelectrodes may adjust a phase of light reflected by the outer
periphery of the cylindrical mesa ll under resonation in the radius
vector direction. The pair of subelectrodes is similar in structure
to that of FIG. 3 to be described later.
[0056] (Second Embodiment)
[0057] As will be seen in FIG. 2, the surface emitting laser of the
second embodiment has a current injection area 60 and a waveguide
area 40 that are concentrically disposed and separated spatially
from each other.
[0058] FIG. 2 is a schematic perspective view of the surface
emitting laser of the second embodiment. Provided on an n-type GaAs
substrate 1 are a vertical cylindrical cavity waveguide 40 having
an axis L vertical to the surface of the substrate 1, a cladding
layer 50 formed around the outer periphery of the waveguide 40 and
a conductive area 60 formed around the outer periphery of the
cladding layer 50. The waveguide 40 formed at the center of the
laser has a periodic structure 42 that causes second-order Bragg
refraction. More specifically, the periodic structure 42 may
comprise a lamination of alternate layers of different optical
characteristics so as to cause light having a wavelength of
.lambda. to cause second-order refraction. A .lambda./4 phase
sifter 47 is provided in the vicinity of the midpoint of the length
of the periodic structure 42 where .lambda. represents a resonation
wavelength. The periodic structure 42 may be a crystal lamination,
for example, of alternate layers of AlGaAs and AlAs or alternate
insulator layers of SiO.sub.2 and TiO.sub.2. The cladding layer 50
having a smaller refractive index than an average refractive index
of the waveguide 40 is provided around the outer periphery of the
cavity waveguide 40 to thereby form a cylindrical mesa. Provided
concentrically around the outer periphery of the cladding layer 50
is the current area 60 comprising a lamination of an n-type AlGaAs
cladding layer 3, an active layer 4 and a p-type AlGaAs layer 5.
The height of the active layer 4 from the substrate 1 is adjusted
so that the active layer 4 is flush with the phase shifter 47 of
the periodic structure 42 because a radiation mode intensity in the
phase shifter 47 is large.(Radiation mode is defined as light
emission normal to the guided mode via the second-order periodic
structure.) In FIG. 2, reference character D denotes an intensity
distribution of a radiation mode radiated from the periodic
structure 42. As shown by the reference character D, the intensity
of the radiation mode has a peak at the phase shifter 47. A high
reflection film 100, for example, made of an insulator is provided
around the outer periphery of the conductive area 60. A p-side
ring-like electrode 20 is provided on top of the conductive area 60
whereas an n-side electrode 21 is provided on the back of the
substrate 1.
[0059] When a bias is applied across the electrodes 20 and 21 to
the surface emitting laser of FIG. 2, the surface emitting laser
emits light in the active layer 4 of the conductive area 60, the
light going and radially returning within the active layer 4 as a
result of reflection by the high reflection film 100, as shown by a
double-headed arrow in the vicinity of the active layer 4 of FIG.
2. When the reflectivity of the outer high reflection film 100 is
high to some extent, the Q of resonation in the radius vector of
the cylindrical mesa increases, and the light is coupled to a
radiation mode from the vertical cavity waveguide 40 to resonate at
the position of the phase shifter 47 within the vertical cavity
waveguide 40. The resonating light is extracted as output from the
upper surface of the waveguide 40. That is, also in this
embodiment, "cross resonation"occurs based on resonations in the
radius vector direction and in the vertical direction of the
cylindrical mesa.
[0060] The present embodiment utilizes coupling via the radiation
mode of the vertical cavity waveguide 40. The waveguide 40 has a
DFB (Distributed FeedBack) structure, so that an AR
(Anti-Reflection) film 48 that suppresses reflection is preferably
provided on the light output surface.
[0061] The surface emitting laser of this embodiment has a similar
concept to that of the invention disclosed in a prior Japanese
Patent Application No. Hei 10-314842. The present embodiment is
unique in that the waveguide is a "vertical cavity type"and the
gain/loss area is not uni-directional, but circulative around the
waveguide.
[0062] Also, in the surface emitting laser of this embodiment the
active layer is far greater in volume than that of the VCSEL of
FIG. 6 to thereby provide an excellent temperature characteristic
and a greatly increased optical output. Especially, in this
embodiment, the waveguide less in temperature change is separated
from the conductive area more in temperature change. This further
improves the stability of the wavelength.
[0063] The p-side ring-like electrode 20 is formed so as to have a
wide contact area on top of the current-injection area 60 that has
enough width. Thus, the contact resistance is reduced and may be
formed with a rough accuracy to thereby facilitate the manufacture
of the electrodes.
[0064] With the surface emitting laser of FIG. 2, the c area 60 is
completely separated from the waveguide area 40, so that the
freedom of the element design increases, advantageously.
[0065] (Modification)
[0066] FIG. 3 is a schematic perspective view of a modification of
the surface emitting laser of the second embodiment. In the laser
of FIG. 3, a p-side ring-like electrode provided on top of the
conductive current-injection area 60 is composed of a pair of
substantially concentric subelectrodes 20A and 20B. By adjusting a
balance between a pair of bias voltages each applied to a
respective one of the pair of subelectrode 20A and 20B, the phase
of light reflected by the outer periphery of the laser due to
resonation in the radius vector direction can be adjusted. A hollow
cylindrical high resistance area 69 is formed between the pair of
ring-like subelectrodes 20A and 20B for electrically insulating
purposes. The high resistance area 69 may be formed, for example,
by injecting proton ions into the conductive area 60.
[0067] As another modification, the periodic structure 42 in the
FIG. 2 or 3 laser may be formed so as to have optically
asymmetrical characteristics with reference to a mid-horizontal
line.
[0068] FIG. 4 is a graph illustrating an asymmetrical distribution
of refractive indexes in the periodic structure 42. In FIG. 4, the
horizontal axis represents a position in the periodic structure 42
in the waveguide direction whereas the vertical axis a refractive
index n in the peripheral structure 42. Such distribution of
refractive indexes can be realized by adjusting the growth
conditions necessary for forming the periodic structure 42. For
example, when a chemical compound semiconductor layer is crystal
grown on the substrate 1, a "grating"structure that changes the
composition of the semiconductor layer gradually may be realized by
adjusting the refractive index distribution in a controlled
manner.
[0069] Asymmetry of the optical characteristic of FIG. 4
corresponds to a braze angle of a diffraction grating and
influences an intensity distribution of the radiation mode in the
waveguide direction as in the phase shift, which is helpful in
matching the active layer with a position where the intensity of
the radiation mode is high.
[0070] (Third Embodiment)
[0071] An optical communication system having any one of the
surface emitting lasers of FIGS. 1-3 will be described as a third
embodiment.
[0072] FIG. 5 schematically illustrates a structure of the
inventive optical communication system. This system comprises a
light emitting element unit 200, a controller 300 therefor, an
optical fiber 400, an photodetector unit 500, and a signal
processor 600 provided after the photodetector unit 500 and
including amplifying elements. The light emitting element unit 200
comprises the surface emitting laser of one of FIGS. 1-3.
[0073] In the FIG. 5 optical communication system, electrical
signals are first sent to the light emitting element unit 200.
These signals are then converted to a laser beam by the surface
emitting laser of the light emitting element unit 200. The later
beam from the surface emitting laser is sent via an optical fiber
400 to the photodetector unit 500 and reconverted to an electrical
signal by a photodetecter of the photodetector unit 500. This
electrical signal is amplified by the amplifying elements of the
signal processor 600. As a result, the amplified electrical signal
is extractable from the signal processor 600.
[0074] As described above, the surface emitting laser of one of
FIGS. 1-3 used in the FIG. 5 optical communications system exhibits
a good temperature characteristic, provides high output and is
inexpensive. Thus, the use of this surface emitting laser in the
light emitting element unit 200 serves to improve the performance
of the FIG. 5 optical communication system and to reduce the whole
cost.
[0075] While the FIG. 5 optical communication system mentioned
above uses the light emitting element unit of the surface emitting
laser of one of FIGS. 1-3, the surface emitting laser may be used
as an optical amplifier or a wavelength selecting element of the
photodetector. While the FIG. 5 optical communication system is
illustrated as transmitting an optical signal, using the optical
fiber 400, the optical signal may be transmitted over a space
without using the optical fiber 400.
[0076] While in the above the embodiments of the present invention
have been described, the present invention is not limited to those
specified embodiments.
[0077] For example, while the use of the GaAlAs/GaAs materials has
been illustrated in the above respective embodiments, the present
invention can use various other appropriate materials to form
corresponding lasers. Such appropriate materials comprise, for
example, various chemical compound semiconductors for Groups III-v
such as InGaAlP, InGaPAs, and BInGaAlN materials, chemical
compounds for Groups II-IV such as ZnSe and ZnS materials, and
semiconductors for Groups IV such as SiC.
[0078] The specified structures of the DBR, the periodic structure
of the vertical cavity waveguide, the conductive area containing
the active layer, and the electrode contacts used in the present
invention may be replaced with other corresponding structures
selected by those skilled in the art to produce advantageous
effects similar to those produced by the above specified inventive
structures. For example, the active layer may have a multiple
quantum well type structure. The cladding layer may have a multiple
quantum barrier structure.
[0079] The number and specified arrangement of those mechanisms can
be appropriately determined by those skilled in the art to produce
similar advantageous effects to those produced by the present
invention. The present invention is applicable in various manners
and expandable without departing from the spirit of the present
invention.
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